The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Referring now to FIG. 1, a tool 10 used in a deposition process for solar device manufacturing may provide radio frequency (RF) power to a chamber 16. The chamber 16 may enclose powered electrodes 19-1, . . . 19-i, . . . , and 19-N (referred to as electrodes 19) and grounded electrodes 20-1, . . . 20-i, . . . , and 20-N (referred to as electrodes 20) associated with respective plasma sources 21-1, . . . , 21-i, . . . , and 21-N (referred to as plasma sources 21).
The tool 10 may include RF sources 22-1, . . . 22-i, . . . , and 22-N (referred to as RF sources 22) that transmit the RF power via transmission lines 24-1, . . . 24-i, . . . , and 24-N (referred to as transmission lines 24). The transmission lines 24 communicate with matching networks 26-1, . . . 26-i, . . . , and 26-N (referred to as matching networks 26) that provide the RF power to respective electrodes 20. RF sensors 28-1, . . . 28-i, . . . , and 28-N (referred to as RF sensors 28) communicate with the transmission lines 24. The RF sensors 28 may provide a reading of received RF power () that is reflected from the plasma chamber 16 and received by the RF sources 22. The RF sensors 28 may also provide a reading of forward RF power ({right arrow over (F)}) that is applied to the plasma chamber 16. Similarly, voltage/current sensors may be used instead of RF sensors 28 to detect voltage and current signals on the transmission lines.
In operation, RF discharges from forward RF power from multiple RF sources 22 may couple through electromagnetic (EM) interaction within the chamber 16 because the electrodes 19, 20 may share a common vacuum and/or ground. RF sensors within the RF sources 22 (not shown) may detect the coupled RF discharges and feed them back so that they are received in corresponding RF sources 22. The reflected RF power may therefore include RF feedback from the plasma chamber 16 that is affected by distortion, such as crosstalk, caused by RF signals from multiple RF sources 22 communicating with multiple electrodes 19, 20 in a plasma chamber 16.
In order to effectively execute a plasma process, it may be desirable to precisely control forward RF power based on reflected RF power/feedback. For example, strict requirements to control forward RF power have evolved as the complexity of solar device manufacturing processes has increased. Consequently, various control techniques are employed to monitor the forward and reflected power.
For example, a typical frequency tuning method operates as follows: the RF sources 22 are turned on and all have a frequency at a starting point, preferably within the same RF band. The RF sources 22 supply forward power to the plasma chamber. A portion of the forward power is reflected from the plasma chamber 16. The reflected power is measured, and the magnitude of the reflected power is stored in memory as received signals. The RF sources 22 then change the RF frequency in one direction. The RF sources 22 again measure reflected power, and compare it with the stored magnitude from the previous measurement.
Based on the change in reflected power, the RF sources 22 again move the frequency. If there is a decrease in reflected power, the frequency is moved in the same direction; if there is an increase in reflected power, then the frequency is moved in the opposite direction. This is continued until the lowest possible reflected power is achieved.